Separating apparatus and method

ABSTRACT

This apparatus for separating a mixture of solid particulate materials of different densities includes an annular vessel with tangentially entering liquid stream injection ports adjacent the bottom of the vessel. The liquid streams entering the vessel under pressure through these ports circulate the mixture and, by reason of different horizontal fluid velocities across the vertical cross-section of the vessel, separate it into fractions according to density. The more dense fraction is removed through the vessel&#39;&#39;s bottom, while the less dense fraction is removed throug a weir in the sidewall of the vessel. A throat above the inlet ports reduces the velocity of the upper part of the circulating mixture and improves classification. Additionally, the injection of small high velocity upward streams of liquid through inlet ports in the vessel&#39;&#39;s bottom tumbles or jigs the heavy fraction, and releases lighter fractions entrained therewith. Water or so-called heavy media can be used as the separating liquid.

United States Patent [191 Loughner Nov. 20, 1973 [21] Appl. No.: 238,038

[52] US. Cl 209/459, 209/172.5, 209/155 [51] Int. Cl B031) 3/00 [58] Field of Search 209/155, 156, 211,

[56] References Cited UNITED STATES PATENTS 1/1941 Nagelvoort 209/172 8/1950 Vogel 209/172.5 X

3,023,903 3/1962 Norton et al. 209/172.5 3,215,272 11/1965 Sweeney 209/211 X FOREIGN PATENTS OR APPLICATIONS 670,825 1/1939 Germany 209/211 Primary Examiner-Frank W. Lutter Assistant Examiner-Ralph J. Hill Attorney-James S. Hight et al.

[5 7 ABSTRACT This apparatus for separating a mixture of solid particulate materials of different densities includes an annular vessel with tangentially entering liquid stream injection ports adjacent the bottom of the vessel. The liquid streams entering the vessel under pressure through these ports circulate the mixture and, by reason of different horizontal fluid velocities across the vertical cross-section of the vessel, separate it into fractions according to density. The more dense fraction is removed through the vessels bottom, while the less dense fraction is removed throug a weir in the sidewall of the vessel. A throat above the inlet ports reduces the velocity of the upper part of the circulating mixture and improves classification. Additionally, the injection of small high velocity upward streams of liquid through inlet ports in the vessels bottom tumbles or jigs the heavy fraction, and releases lighter fractions entrained therewith. Water or so-called heavy media can be used as the separating liquid.

18 Claims, 5 Drawing Figures PMENIED REV 20 I973 SEPARATING AEPPARATUS AND METHOD The present invention relates to a method and apparatus for separating or classifying mixed solid particulate materials which have different densities. Although it is useful for a wide range of mixed materials, the invention is especially useful for classifying and separating coal from slate or other rocky materials, and it is explained hereinafter primarily in relation to such use. From the following description those skilled in the art will recognize that it is also applicable to the separation of materials such as gold, silver and other ores, so long as there are differences in specific gravity between the various materials in the mixture.

It is known to use apparatus such as that disclosed in Norton et al. US. Pat. No. 3,023,903 in connection with the separation of a mixture of materials of different densities. Such apparatus requires use of a heavy liquid media, that is, a liquid of intermediate density in which the less dense fraction will float and in which the more dense fraction will sink. Where coat is to be separated from rock, the Norton et al. patent teaches the use of water containing suspended magnetite as the heavy media liquid. The coat fraction floats on this liquid and is taken off the top; the rock fraction sinks and is taken off through the bottom. Such techniques are called float-and-sink separations. Paddles or sweeps are required by Norton to provide the mixing agitation necessary to maintain the particles of the heavy media in uniform suspension, and to move the sink" material around the bottom of the vessel.

It is a disadvantage of such apparatus that it requires a heavy media to effect the separation. This entails suspending and later removing the magnetite or other density increasing additive. The cost of the additive and its recovery are substantial. Moreover, the mechanical stirring mechanism is expensive and prone to wear from the abrasion inherent in stirring rock-coal mixtures.

While it has been suggested in US. Pat. No. 1,937,190 that liquid introduced through tangential ports may be utilized to distribute an upward rising agitation liquid in a system utilizing a dense medium for float-sink operation, I am unaware of any patent which utilizes either paddles or jets to impart a velocity to a light liquid medium such as water, that is sufficient in itself to classify materials throughout a vertical crosssection of the vessel.

Other prior art systems for classifying materials by reason of their different densities are shown in US. Pat. Nos. 1,470,531 and 1,167,638. The function of these systems depends on the concept that where a mixture of materials is introduced into a fluid moving in a vessel, lighter particles will be carried a greater distance from the infeed point than will the heavier particles and concentrations of like particles may thus be removed from the bottom of the vessel at corresponding distances from the infeed point. Such systems have the disadvantage that means must be provided to maintain a lateral flow of the medium (1,470,531 or to maintain uniform velocities throughout the medium (1,l67,638). and that the carry depends on point of entry, particle size and shape and other non-constant factors.

My invention provides a separating method and apparatus which does not require heavy media but rather can utilize a light medium such as water, without any density-increasing fraction additive. By injecting this liquid under pressure in a certain way into a separation vessel of a certain type, the liquid can of itself cause classification of the various fractions of a mixture within the medium. The denser materials will stratify or concentrate in the lower portion of the vessel while the less dense materials will stratify or concentrate in the upper portion of the vessel. This is achieved without paddles or stirrers or other mechanical mixing aids.

In this invention the separation is achieved by differential liquid velocities while circulating in an annular direction. It does not depend on the float-sink" concept. It is not required in the utilization of my invention to provide a medium with a density generally between the densifies of the particles to be separated, although such a media can be used if it is already available.

The apparatus of my invention includes an annular chamber or separation vessel which is defined by a bottom wall and inner and outer sidewalls. Several liquid inlet ports enter the vessel tangentially through the sidewall, adjacent the bottom. In use a liquid such as ordinary water is injected under pressure through these so that it enters the tank as a high speed jet or stream, and it imparts a circulating force to the solids of the mixture.

It is important that the velocity of the injected medium be greater at a bottom portion of the vessels cross-section than at the top thereof. To that end, the liquid is introduced into the vessel adjacent the bottom, but its velocity is desirably reduced in the upward direction by an annular throat of lesser area above the ports, or by an increase in vessel width above the inlet ports.

The rate at which the medium is injected is adjusted or set to cause the solid particles to move around the vessel at a speed at which they classify according to density. Typically this will be in the range of about 4 to 6 feet per second, but it will vary with the nature of the mixture to be separated, and in any case it is readily determined by use of throttling valves to change the liquid flow rate. In a short time (a few circulations), the lightest density fraction is concentrated at the top and the heaviest fraction at the bottom, a not yet separated mixture of light and dense fractions in a middle zone. A weir is provided in the top portion of the inside wall of the vessel and the separated materials of a light density spill out through this and are taken off for further treatment. A discharge opening is provided at a location in the bottom wall of the vessel for the removal of materials of greater density.

Additional but much smaller fluid inlet ports are desirably provided in the bottom wall of the vessel. These enter the tank at an angle to the direction of fluid circu lation. They have the effect of jigging or breaking up clumps of material in the lower portion of the vessel and the updraft they establish facilitates release and separation of the less dense materials.

A constriction which defines a throat-like horizontal zone of reduced annular area, is mounted on one or both of the sidewalls around the vessel above the inlet ports. This reduces the velocity of the medium above the throat, relative to the velocity below the throat.

Altemately, the inner wall of the vessel may be set back or stepped above the inlet ports. This reduces velocity of the medium in the top portion of the vessels cross-section and thereby enhances stratification of the mixture.

It has been the main object of my invention to provide apparatus for classifying solid materials of varying densities in a mixture, without use of either a heavy media or mechanical sweeps, or paddles or stirrers.

It is an advantage of the invention that it can eco nomically separate even a small concentration high quality coal from admixed fine rock refuse, and thereby can recover coal from a refuse pile which otherwise would be of no value and which otherwise would be wasted.

These and other objects and advantages will become readily apparent from the following detailed description and from the drawings in which:

FIG. 1 is a side view (partly in section) of a preferred form of apparatus in accordance with the invention, and shows the positions of the tangential inlet ports and of the discharge means for the heavy fraction of the material;

FIG. 2 is a top plan view showing the tangential inlet ports, the annular separation chamber, the infeed means, and the weir discharge means;

FIG. 3 is a vertical section taken on line 33 of FIG. 2 and shows the separation of the materials therein including the takeoff of the light fraction over the weir;

FIG. 4 is a cross sectional view of an alternative annular separation vessel which has a velocity reducing set-back or step in the inner wall thereof; and

FIG. 5 is a cross sectional view of another modified embodiment.

Referring to the drawing in detail, FIG. 2 shows a preferred form of separation apparatus at 10. it includes an annular preferably circular separation vessel 11 which is defined by a vertical inner wall 12, a vertical outer wall 13, and a flat bottom wall 14!- (FIG. 3). An annular horizontal flange or splash guard 15 is mounted to outer wall 13 at the upper edge thereof and partially extends over the top of the vessel Ill. The circular annular shape illustrated is preferred, but other closed annular shapes are usefui, including oval vessels.

A plurality of liquid injection inlet ports 16 are provided about the outside wall of vessel llll. The ports is (five in number in the preferred embodiment) are each connected to an annular supply manifold 17 by branch conduits 18. Supply manifold 337 has a flange 19 for connection to a source (which may be conventional and is not shown) for supplying a large volume of liquid, suitably water, to the manifold under pressure. Pinch or throttling valves 21 may be provided in conduits 18 to permit the velocity of the liquid to be adjusted as desired. As stated, the liquid used for separation may be water, even though its density is less than that of both the coal and slate or other materials to be separated. One of the advantages of this present method is that the separation can be made without use of a heavy media, containing sand, magnetite or the like.

The ports 16 open tangentially to vessel Ill adjacent the tank bottom 14. While these openings to must be adjacent the tank bottom, they can be provided in either the side wall (preferably the outer side wall 13) as shown in the drawing, or they can be formed through the bottom 14 itself, such that flow enters tangentially but at a slight upward angle. The shape of the ports may be oval as shown, but circular or slot configurations are also useful. The velocity of liquid at the point of injection is typically about 4-5 feet per minute, although this will depend on particle size and type. The kinetic energy of the liquid imparts circulating movement (in the direction of arrow A in the embodiment shown) to the solid particles. Another tangential inlet port 22 is supplied with the liquid preferably from a separate pressure through conduit 23, and also enters the tank adjacent the bottom. The liquid from this conduit 23 inpinges on the particulate solid material just at the point where the latter is introduced into the vessel, and aids in starting it circulating in the cirection of arrow A. (The ports 16 may be reversely oriented, to provide clockwise flow; the apparatus does not depend upon or require a Coriolus effect). The flow of the medium through this port 22 may be controlled in order to vary the velocity of this medium in the vessel.

Additional small bottom ports 25 are connected to the source for supplying fluid under pressure, by way of headers or branch lines 26 (see FIG. 1). The ports 25 open through the bottom wall 14 of vessel 11 and inject streams of the medium obliquely upwardly, with a component of motion generally in the direction of arrow A, at an obtuse angle with respect to the circulating material.

A solid material infeed means such as a conveyor 30 is utilized to feed a mixture of materials such as raw coal, slate and rock into the vessel 11. The point of infeed is above the liquid level in vessel 11, preferably just downstream of position at which port 22 injects liquid, so that incoming solids will rapidly be set in motion without settling.

For withdrawing the upper portion or strata of material from the vessel 11, a weir or opening (see H68. 2 and 3) is provided in the inner wall 12 of the vessel ill. The bottom edge 36 of weir 35 is located below the operating level 20 of the liquid so that a portion of the liquid, and ther materials it contains, can flow continuously through weir 35 onto chute or conveyor 37 for transfer to dewatering and/or screening apparatus. The vertical location of the bottom 36 of weir 35 determines the depth of the strata that is removed, and the weir may be adjustable, thereby to change the average composition of the material removed; in general, the higher the weir, the more uniform will be the fraction drawn over it.

For removing the lower layers or strata of material from vessel ll, a bottom discharge means 40 is provided, as best seen in FIG. I. The discharge 40 includes an outlet port 4i in the bottom wall 14 of vessel 11. The port 41 may be controlled by a rotary star valve 42 of conventional construction to limit the rate of solids withdrawal. Valve 42 is operated rotationally for positive release of the heavy density material downwardly from vessel lll into a chamber 43 where it is picked up and removed by a bucket or other conveyor 44. A liquid outlet 45 is provided to maintain a strong flow through the outlet. Baffle do inhibits the flow of discharged materials through liquid outlet 45. However, it should be noted that instead of using a star valve, the rate of material removal can be controlled by varying the speed of conveyor 49.

Constrictions or restrictors 50, (MG. 3) are preferably provided on one or both inside walls of the vessel, above the ports 16. These define an annular horizontal throat area 51 between them which is of lesser width than that of the vessel 11 immediately above and below the restrictors. These have been found effective to reduce the velocity of the fluid toward the top of the vessel, so that liquid in the lower chamber or zone 57 circulates at a faster rate than liquid in the chamber or zone 58, above the restrictors 50, 50. In the embodiment shown the contrictions 50, 50 are right angular flanges that extend substantially continuously around the inside surfaces of both inner and outer walls 12 and 13, as shown in FIG. 3. Throat 51 is above the inlet ports 16.

A modified constriction of an annular separating vessel 55 in accordance with the invention is shown in FIG. 4. This vessel 55 has ports similar to the preferred embodiment, as designated by like numerals, but differs from that shown in FIG. 3 in that inner wall 12 is set-back or stepped outwardly as at 56. A constriction 50 is provided on only one wall, because the set-back of itself reduces velocity in the upper chamber 58. Again the inlet ports 16 are located adjacent the bottom of vessel 11, below throat 51 (FIG. 1), the velocity of the medium is greater in the zone below constriction 50, and the liquid in the upper portion 58 of the tank moves more slowly than that in the bottom portion. The combination of the set-back or step 56 and the constriction 50 shown in FIG. 4 results in a vessel crosssection of a greater width across the top zone 58 than across the bottom zone 57. The particular shape of the constrictors illustrated in FIGS. 3 and 4 is not critical and other shapes may be used to slow the rate of fluid movement in the upper zone. While the drawings disclose an embodiment having two vertical zones separated by an intermediate constriction or throat area, it is contemplated that two or more vertically spaced flow restrictors may be used, to provide a greater velocity differential between the uppermost and lowermost zones of circulating fluid and thereby to provide longer settling time.

In carrying out the method of this invention, a mixture of solid particles of different densitiessuch as raw coal, rock and slate mixture-is fed into vessel 11 via infeed 30. Water (without a density increasing additive) is injected through ports 16 to fill vessel 11 to the level designated at 20, and is driven around vessel 11 at a predetermined velocity in the direction of arrow A due to the tangential position of ports 16. The particles tend to assume the velocity of the liquid.

The coal, rock and slate have different density and they stratify in the circulating liquid, the lighter density coal concentrating at the top layer and the greater density rock and slate concentrating at a bottom portion.

I have discovered that the lower liquid velocity in top zone 58 does not maintain the heavier rock and slate in suspension and that those materials stratify at lower areas, where the greater velocity circulates them over the outlet 41. The greater velocity in the lower zone 57 facilitates the release of any coal entrained therein, in the lower strata, which then moves toward the top zone. In this connection, it is theorized that the effect of the differential in velocities between the upper and lower zones is to provide agitation of the particles, such that natural classification can occur, in accordance with particle densities. It is thought that the fluid movement does not suspend, sweep, or float light particles upward. The fluid may be injected in a horizontal direction without significant vertical component of motion.

The injection of water through the bottom ports 25 further agitates, tumbles, or jigs ther material moving over the bottom of vessel 11, and improves release of lighter density particles from such entrainment.

As the coal is stratified at the top portion of the vessel, it is discharged through weir 35 onto takeoff chute 37. The height of the weir can be adjusted to control the depth of the removed fractions; in general, the higher the weir the cleaner the separation but the slower the rate of separation. Generally the weir will be adjusted to take off the thickest layer or strata that meets required coal specifications. The deeper or thicker the strata removed, the greater the chance of removing a portion of the rock or slate with the coal.

The rate of discharge of the heavier fraction rock and slate at the bottom of the vessel 11 may be varied by changing the rotational velocity of the star valve 42.

In the alternate embodiment of FIG. 4, the particle velocity is still less throughout the top zone 58 in comparison to the bottom zone 57, and there is a relatively sharp velocity gradient at the level of restrictor 50. This enhances the stratification rate. This construction is particularly useful where the raw mixture comprises a relatively large portion of coal and a lesser portion of rock, slate or refuse material, and effects an increased output of high quality coal. Accordingly, this embodiment is especially useful for treating mined coal, as opposed to coal in scalped refuse piles.

Weir 35 is preferably located in inner wall 12. I have found that the heavier fraction (e.g., materials of greater density than coal) tends to be circulated adjacent the outer wall 13 by the centrifugal force of rotation. Placement of the weir 35 in the inner wall 12 has been found to provide a cleaner light fraction than a weir in the outer wall.

The apparatus and method are not limited to use with clear water. I-Ieavy media (containing magnetite, etc.) can be used, if economics favor it, for example, where the mixture is rich in coal. For recovery of coal from refuse piles, heavy media is expensive to prepare and recover; and its use is not necessary.

Following is a specific example of the practice of the invention to separate coal remaining in an otherwise worthless refuse pile:

EXAMPLE An apparatus constructed in accordance with my invention has an annular width of 10 inches between the inside and outside walls at the bottom, and a 5 inch setback on the inner side wall which increased diameter above it to 15 inches. The diameter between the outside walls is approximately 10 feet. Manifold 17 is a 10 inch diameter pipe, and water is supplied to it at a rate of 2,400 gallons per minute. Of this total gallonage, about 1,400 gallons per minute are delivered to five branch conduits 118 which are 3 inches in diameter, and spaced equally (including inlet 23) around the tank immediately adjacent the bottom of the tank. A 4 inch main 23 is supplied at a rate of 1,000 gpm from the manifold 17. The bottom port 41 is 10 inches wide X 2 feet long, and is spaced just upstream of the port of entry of the conduit 23. The infeed material was charged through the open top of the tank, directly above the point of entry of the conduit 23.

A mixture containing approximately one-third coal and two-thirds refuse by weight is charged to the tank by conveyor 30, at a rate of tons per hour. The largest particles are about 2 inches in maximum dimension. Particles circulate in the upper zone 58 at a rate of approximately 3 to 5 feet per second; in the lower zone the rate of particle movement is S to 7 feet per second.

A mixture of coal and small and light refuse particles is withdrawn over the weir at a rate of 125 tons per hour. The weir depth is 14 inches. The over-the-weir mixture is then dewatered and simultaneously separated from the fine 7 1 X fraction on a small single deck vibrating screen. This provides an output of clean coal of tons per hour, in the 23 X A size range. The A X 0 fraction from the screen can be passed to a hy drocyclone or Diester table for further separation and will typically yield about tons of coal and 65 tons of refuse. Rock is taken off through port 41 at a typical rate of about 30 tons per hour.

Water was injected through half inch ports 25, set in groups of five at two angularly spaced locations in the bottom of the tank, at a rate of about 10-20 gallons per hour per port.

The restrictors in the tank may comprise a two inch wide annular ring welded to the walls of the tank 8 inches above the tank bottom, and 4 inches above the top of inlet 16. Without the restrictor, both the rate and the quality of the separated coal fraction are significantly poorer.

FIG. 5 illustrates another modified embodiment which differs from that described above in that a stronger, more uniform fluid upflow is provided from the bottom of the tank. A baffle plate 60 is mounted in the tank, spaced above the bottom 61. Plate 64) is in the form of an annular ring or arc and joins the tank sidewalls 62 and 63 along its inner and outer peripheries, respectively.

Plate 6% presents a large number of apertures 65; these suitably are about A inch in diameter and are spaced about 80 per square foot but these values are not critical. Water or other liquid media is supplied through a line 66 into the chamber 67 below plate 60, and this wells or gushes up through the ports 65 into the separating chamber above the plate. This upflow has the effect of improving the rate and quality, and provides a more uniform action than the jets at 25 in the embodiment first described. Plate 60 is desirably spaced below the rim or throat 63, and below the tangential inlets 16 so that they function as previously described. A heavy fraction outlet port (which may be similar to the port ll previously described) opens through the baffle plate 66.

While I have specifically described a preferred form of my invention, various modifications and variations will become readily apparent to those of ordinary skill in the art, and I intend to be bound only by the appended claims.

I claim:

1. Apparatus for separating a mixture of solid particles into fractions according to density, comprising,

an annular vessel defined by bottom, inside and outside walls,

means for introducing said mixture into said vessel at the top thereof,

a plurality of liquid inlet ports tangentially entering said vessel at spaced locations adjacent said bottom wall,

means located above said liquid inlet ports for reducing the velocity of said liquid in an upper portion of said vessel relative to the velocity of said liquid in a lower portion of said vessel,

means for supplying liquid under pressure to said vessel through said ports at sufficient velocity to cause said fluid to circulate said mixture around said vessel and for classification to occur in the mixture while so circulating,

said vessel having an outlet port at the bottom thereof for discharge of a heavier fraction, and

a weir in one wall of said vessel over which a lighter fraction can be withdrawn.

2. The apparatus of claim 1 wherein said means for reducing comprises a set-back in at least one of the sidewalls such that said vessel has a greater width above said set-back than it does below the set-back, the setback thereby defining an upper zone and a lower zone of different annular widths.

3. The apparatus of claim I wherein said means for reducing comprises a restrictor on at least one of the sidewalls above said ports, said restrictor defining a horizontal throat area which is relatively narrow in annular width as compared to the zones above and below it,

said restrictor reducing the velocity of particle movement in the upper zone in relation to that in the lower zone.

4. The apparatus of claim 3 wherein said restrictor is a flange extending circumferentially within said vessel around at least one of the sidewalls.

5. The apparatus of claim 1 wherein the sidewalls are substantially vertical, and said vessel is U-shaped in cross-section.

6. The apparatus of claim l which includes a plurality of secondary fluid ports entering said vessel at spaced positions through the bottom wall thereof, for directing fluid under pressure into said vessel upwardly at an angle to the direction of circulation therein, and

means for injecting fluid through said secondary ports at a velocity sufficient to open clumps of heavier fraction material passing thereover and to release lighter fraction materials entrained in such clumps.

7. Apparatus in accordance with claim 1 wherein a supply manifold surrounds the outside wall of said vessel adjacent the top thereof, and

wherein a plurality of inlet tubes extend anguiarly downwardly between said manifold and said ports to convey fluid from said supply manifold to said ports.

8. Apparatus in accordance with claim 1 wherein a liquid inlet port enters said vessel through the outside wall thereof adjacent the bottom wall, at such point that liquid being injected through it impinges on mixture introduced into said vessel directly above it.

9. Apparatus in accordance with claim 1 including means for varying the rate of discharge of said heavier fraction.

10. Apparatus in accordance with claim 1 wherein said weir is formed in the inside wall of said vessel.

11. Apparatus in accordance with claim 1 wherein said ports enter said vessel through the outer sidewall thereof, closely adjacent the bottom.

12. A method of separating a mixture of solid materials of different densities into a more dense fraction and a less dense fraction, said method comprising the steps of,

feeding said mixture from above into a liquid which is circulating in an annular vessel,

simultaneously tangentially injecting multiple streams of said liquid into said vessel at a lower portion thereof so as to circulate said mixture around said vessel,

reducing the velocity of circulation in an upper zone in comparison to the velocity in a zone adjacent the bottom of said vessel sufficiently to cause less dense materials to concentrate adjacent the top of the vessel and more dense materials to concentrate adjacent the bottom of said vessel,

removing a lessdense fraction adjacent the top of said vessel, and

removing of a more dense material from a lower zone of said vessel.

13. The method of claim 12 wherein said liquid consists of water.

14. The method of claim 12 wherein said mixture is a mixture of coal and rock.

15. The method of claim 12 wherein said mixture is refuse coal and rock.

16. The method of claim 12 wherein the velocity of particles in the top zone is about 3 to 5 feet per second.

17. The method of claim 16 wherein the velocity of particles in the lower zone is 5 to 7 feet per second.

18. The method of claim 12 which further includes injecting streams of liquid upwardly through the bottom of said vessel at velocities sufficient to release light particles entrained in clumps of heavy particles passing thereover. 

1. Apparatus for separating a mixture of solid particles into fractions according to density, comprising, an annular vessel defined by bottom, inside and outside walls, means for introducing said mixture into said vessel at the top thereof, a plurality of liquid inlet ports tangentially entering said vessel at spaced locations adjacent said bottom wall, means located above said liquid inlet ports for reducing the velocity of said liquid in an upper portion of said vessel relative to the velocity of said liquid in a lower portion of said vessel, means for supplying liquid under pressure to said vessel through said ports at sufficient velocity to cause said fluid to circulate said mixture around said vessel and for classification to occur in the mixture while so circulating, said vessel having an outlet port at the bottom thereof for discharge of a heavier fraction, and a weir in one wall of said vessel over which a lighter fraction can be withdrawn.
 2. The apparatus of claim 1 wherein said means for reducing comprises a set-back in at least one of the sidewalls such that said vessel has a greater width above said set-back than it does below the set-back, the set-back thereby defining an upper zone and a lower zone of different annular widths.
 3. The apparatus of claim 1 wherein said means for reducing comprises a restrictor on at least one of the sidewalls above said ports, said restrictor defining a horizontal throat area which is relatively narrow in annular width as compared to the zones above and below it, said restrictor reducing the velocity of particle movement in the upper zone in relation to that in the lower zone.
 4. The apparatus of claim 3 wherein said restrictor is a flange extending circumferentially within said vessel around at least one of the sidewalls.
 5. The apparatus of claim 1 wherein the sidewalls are substantially vertical, and said vessel is U-shaped in cross-section.
 6. The apparatus of claim 1 which includes a plurality of secondary fluid ports entering said vessel at spaced positions through the bottom wall thereof, for directing fluid under pressure into said vessel upwardly at an angle to the direction of circulation therein, and means for injecting fluid through said secondary ports at a velocity sufficient to open clumps of heavier fraction material passing thereover and to release lighter fraction materials entrained in such clumps.
 7. Apparatus in accordance with claim 1 wherein a supply manifold surrounds the outside wall of said vessel adjacent the top thereof, and wherein a plurality of inlet tubes extend angularly downwardly between said manifold and said ports to convey fluid from said supply manifold to said ports.
 8. Apparatus in accordance with claim 1 wherein a liquid inlet port enters said vessel through the outside wall thereof adjacent the bottom wall, at such point that liquid being injected through it impinges on mixture introduced into said vessel directly above it.
 9. Apparatus in accordance with claim 1 including means for varying the rate of discharge of said heavier fraction.
 10. Apparatus in accordance with claim 1 wherein said weir is formed in the inside wall of said vessel.
 11. Apparatus in accordance with claim 1 wherein said ports enter said vessel through the outer sidewall thereof, closely adjacent the bottom.
 12. A method of separating a mixture of solid materials of different densities into a more dense fraction and a less dense fraction, said method comprising the steps of, feeding said mixture from above into a liquid which is circulating in an annular vessel, simultaneously tangentially injecting multiple streams of said liquid into said vessel at a lower portion thereof so as to circulate said mixture around said vessel, reducing the velocity of circulation in an upper zone in comparison to the velocity in a zone adjacent the bottom of said vessel sufficiently to cause less dense materials to concentrate adjacent the top of the vessel and more dense materials to concentrate adjacent the bottom of said vessel, removing a less dense fraction adjacent the top of said vessel, and removing of a more dense material from a lower zone of said vessel.
 13. The method of claim 12 wherein said liquid consists of water.
 14. The method of claim 12 wherein said mixture is a mixture of coal and rock.
 15. The method of claim 12 wherein said mixture is refuse coal and rock.
 16. The method of claim 12 wherein the velocity of particles in the top zone is about 3 to 5 feet per second.
 17. The method of claim 16 wherein the velocity of particles in the lower zone is 5 to 7 feet per second.
 18. The method of claim 12 which further includes injecting streams of liquid upwardly through the bottom of said vessel at velocities sufficient to release light particles entrained in clumps of heavy particles passing thereover. 